The present invention relates to the field of heat transfer printing by means of transfer sheets having sublimable dyes thereon.
It is known to employ blocking agents in conjunction with a transfer sheet which contains a heat transferable dye in order to immobilize portions of the dye during the printing process. See, for example, U.S. Pat. No. 4,171,202 issued to Carl E. Sideman and Thomas E. Lewis on Oct. 16, 1979. In the Sideman et al patent a blocking agent is employed which is chemically reactive with at least one of the heat transferable dyes, with disclosed exemplary blocking agents including hydrosulphites, isocyanates, aminoplasts, epoxy resins, melamine-based compounds and glyoxal-based compounds.
Similarly, Japanese Laid-Open Specification No. 40310/1978 of Apr. 12, 1978 discloses a transfer sheet which includes both a heat transferable dye layer and an anti-dyeing layer which serves to block transfer of the dye in specific areas. The blocking agents which are disclosed comprise metallic compounds such as compounds of the metals chromium, iron, copper, nickel and cobalt.
More specifically, the heat transfer printing process such as described in the noted Sideman et al patent is effected by heating the transfer sheet and the material to be printed, both of which have been placed together, at a temperature of from about 160.degree. to 220.degree. C., for a sufficient period of time to allow the heat transferable dye on the transfer sheet to sublime or vaporize and then to diffuse and penetrate into the material which is being printed. The blocking agent is deposited on a portion of the dye and during the heating process undergoes a chemical reaction with at least a portion of the dye upon which it has been deposited. The reacting dye is subsequently unable to sublime or vaporize and therefore is not transferred. The extent of this reaction depends upon the nature of the dye used and the concentration of the blocking reagent. By varying the concentration, either complete blockage of dye transfer or partial blockage to produce shading effects can be obtained.
A limitation on the process described in U.S. Pat. No. 4,171,202 has been that designs transferred by the process are limited to some combination of (1) the underlying color of the material being printed; (2) the color of the dye transferred; and (3) some lighter shade of the dye transferred achieved by partial blocking. These limitations arise because the blocking agents heretofore known have the ability to react with a broad range of dyes, making it very difficult to layer dye colors on the same substrate and selectively transfer one while blocking the other. Thus many desirable fabric patterns could not be accomplished by the process heretofore known. For example, patterns wherein many small dots of one color appear in a background of a second color could not be achieved in cases where each color had to be printed onto a colorless (white) background. Two color printing in such a case is also difficult or impossible because of the registration tolerances required.
It is thus desirable to provide a heat transfer sheet which includes a heat transferable dye and a blocking agent which is selective as to its blocking activity with regard to various classes of dyes to enable greater control of dye transferability to be achieved.